This invention relates to a medical treatment device for treating an undesired solid formation in a living body, by bombardment of the formation with a pulsed liquid or semi-liquid jet.
The invention has been developed primarily with a view to treating undesired solid formations in human patients, examples of such undesired formations including stones (especially kidney stones), though other growths and deposits in the human body may be treated, including natural or artificial deposits, such as cements used in bone joint replacements.
The term "treating" used herein is intended to include fragmentation, total removal or cleaning.
A medical treatment device according to the invention is particularly suitable for use in the removal of kidney stones, by progressive bombardment of the stones so as to cause fragmentation of the stones into small fragments which can then be removed by a surgeon by any convenient means, such as an endoscope.
Kidney stones generally form because of a metabolic abnormality which increases the concentration of crystaloids in the urine. The stones consist mainly of inorganic salts of calcium although salts of uric acid, oxalic acid and phosphoric acid may also occur depending on the long term acidity or alkalinity of the urine. The stones may vary in size from grits up to golf ball dimensions and in consistency from soft or cheesy to hard and brittle. In the majority of cases, the stones are passed spontaneously along the urethra but many of the larger ones lodge in the kidney pelvis or calyses where they would, in general, require open surgery to remove them. Such surgery might involve extracting the stones through a small hole or, in severe cases, a partial or total nephrectomy, that is a removal of the kidney. Whichever method is used the recovery and convalescene period is that required for many other major operations.
A general surgical procedure which has been used for some years is minimal intrusion surgery or endoscopic surgery in which endoscopes having tubes of small dimensions are introduced into the body through what amounts to a small puncture hole in the muscles, skin and organ being operated on. It would be advantageous to be able to treat kidney stones using endoscopic techniques. An endoscope can be introduced into the vicinity of a kidney stone but such a stone may be too large to be extracted through the endoscopic tube. Such stones would have to be broken up into smaller pieces in order that they could be extracted.
The invention seeks to provide a medical treatment device which is able to be used, inter alia, in the removal of a kidney stone, in that it is concerned with the bombardment of the kidney stone with a pulsed high velocity charge of low density material which cleaves the stone into fragments which may be then removed.
The low density material of which the charge is composed may be water or other suitable liquid, or a liquid based material which is rendered semi-liquid or at least partly "solid" by introduction of an additive to the liquid. One preferred example of the latter comprises a "gel" which is obtained by adding controlled amounts of gelatine to water, typically in an amount of a few percent. A gel can be prepared for use in a device according to the invention, which takes any desired form within the range of a liquid through to a solid.
A kidney stone may be cleaved by a high velocity short duration charge because of the intense pressure created as a result of impact between the charge (when it includes a liquid component) and the stone. This is due to the compressible behavior of the charge in the initial stages of the impact. The pressure, for a rigid target, is given by the equation P=pCV where p is the liquid density, C is the shock wave velocity in the liquid and V the impact velocity. The pressure is referred to as a "water-hammer" pressure and continues as long as the contact area between the impacting charge and the solid expands supersonically with respect to waves in the liquid. The duration of this stage depends on the impact velocity and the radius of curvature of the head of the charge but is generally in the range 0.1 to 1 microseconds.
The short duration of the liquid charge has the advantage that there is minimal movement of the impacted body compared with, for instance, impacting a kidney stone by means of a solid projectile.
The high velocity charge or "jet", after it emerges from the nozzle or orifice, remains coherent in air for typically a few centimeters, the precise distance depending on many factors, including the jet velocity, diameter, density and surface tension. The coherence distance is also dependent on the density of the fluid traversed by the jet. Within a more dense fluid such as a liquid the distance over which the jet remains coherent is reduced. This is an advantage in treating kidney stones because if the stone is missed by the jet, the damage capability of the jet at surfaces further away is considerably reduced.